I remember having this same question, confused me for a while as well.

Your right on the breaking it up into parts:

What is the energy of a D meson at rest? You might have to look this up online, when I solved it this information was supposed to be given (they didn't give the number, we had to look it up).
try using Einsteins Equation

Now After:
What is the rest energy of a K meson and Pi meson? This combined rest energy should be less than the einitial D meson energy. The difference in energy must have been converted to kinetic energy.

Use conservation of energy and momentum to relate the four-momenta of the three particles. The trick is to isolate the four-momentum of the K meson on one side and then square the resulting equation. That will let you solve for the energy of the pion in terms of the masses. Once you know the energy, you can find the velocity.